The Spring Security JWT token authentication flow

 The Spring Security JWT token authentication flow involves several steps, typically illustrated through two main phases: Login/Token Generation and Resource Access/Token Validation.

1. Login/Token Generation Flow:

Client Sends Credentials:
The client (e.g., web browser, mobile app) sends user credentials (username and password) to the Spring Boot application's authentication endpoint.
Authentication Controller:
The application's authentication controller receives these credentials.
AuthenticationManager:
The controller delegates the authentication process to the AuthenticationManager.
AuthenticationProvider:
The AuthenticationManager uses an AuthenticationProvider (often integrated with UserDetailsService) to validate the credentials against a user store (e.g., database).
Successful Authentication:
If credentials are valid, the AuthenticationProvider returns an Authentication object.
JWT Generation:
A JWT utility class or service generates a new JWT containing user details (claims) and signs it using a secret key.
Token Sent to Client:
The generated JWT is returned to the client in the authentication response.

2. Resource Access/Token Validation Flow:

Client Sends JWT:
For subsequent requests to protected resources, the client includes the obtained JWT, typically in the Authorization header as a "Bearer" token.
Spring Security Filter Chain:
The request enters the Spring Security filter chain.
JwtAuthenticationFilter:
A custom JwtAuthenticationFilter (or similar filter) intercepts the request early in the chain.
Token Extraction and Validation:
This filter extracts the JWT from the header and validates it using the secret key (e.g., signature verification, expiration check, claim validation).
User Details Loading:
If the token is valid, the filter extracts user details (e.g., username) from the token's claims and loads the full UserDetails object, often via a UserDetailsService.
SecurityContextHolder:
An Authentication object is created with the loaded UserDetails and set in the SecurityContextHolder, making the user authenticated for the current request.
Authorization:
Subsequent filters or controllers in the chain can then perform authorization checks based on the authenticated user's roles or authorities.
Access Protected Resource:
If authorization is successful, the request proceeds to the intended protected resource.
Token Invalid/Missing:
If the token is invalid, missing, or expired, the JwtAuthenticationFilter or a subsequent filter will reject the request, typically returning an unauthorized (401) or forbidden (403) status.